Until recently, most combined cycle power plants were optimized solely for base load conditions. Little thought was paid to start-up operations because it was expected that, during the early years of a power plant's life, the combined cycle plant would be operated continuously because it was more efficient than older power plants.
Because start-up operation under this assumption was a small percentage of overall plant operation, inefficiencies, emissions, fuel consumption, thermal stresses, and other aspects of start-up and reduced-load operations were not considered significant in the overall picture. A conventional combined cycle power plant is generally started up using the following procedures:
First, a Combustion Turbine Generator (CTG) is started, accelerated to firing temperature, fired, accelerated to synchronous speed, synchronized, and then brought to a minimum load. In practice, a CTG control system automatically controls this sequence. Once minimum load is achieved, the operator can increase CTG load. Generally, the operator would prefer to minimize CTG load at the beginning of the startup to reduce the amount of heat that needs to be rejected by a Heat Recovery Steam Generator (HRSG). However, sometimes, the CTG is restricted from operating at low loads for extended periods of time because of mechanical restrictions or the desire to reduce air emissions, which are usually much higher at low loads.
Before the CTG can be fired, it must be purged to remove potential fuel vapor collecting in the CTG and HRSG after the unit is shut down. Purging is usually accomplished by accelerating the CTG to part speed and holding at part speed until a specified number of air changes occur in the HRSG and CTG. As there is much more volume in an HRSG than a CTG, a “simple cycle” CTG (without an HRSG) only requires purging for several minutes, while a CTG with HRSG requires purging for up to half an hour.
While purging, significant auxiliary energy is used to crank the CTG. In addition, the exhaust leaving the CTG is close to ambient temperatures. If this purge exhaust enters a hot HRSG, it will cool off the HRSG. Steam can actually condense in the superheater section (the part of the HRSG that increases steam temperature), causing stress in the superheaters. Any condensate produced in the superheater must be drained to prevent slugs of condensate being pushed into the steam pipes.
The hot exhaust leaving the CTG during this sequence passes through the HRSG, where it heats up the water in the HRSG and raises pressure in the steam drums. When steam pressure is slightly above atmospheric pressure, the rate of the heat-up can be controlled by venting steam from the drums and at the outlet of the HRSG. To minimize thermal stress on the high pressure (HP) drum, HRSG vendors typically prefer to limit the heat-up rate of this drum's shell to about 200° F. per hour. However, this rate is usually exceeded before venting occurs.
CTGs are constant flow machines—once they reach design speeds, the exhaust mass flow is fixed within narrow limits. Because a significant amount of energy is used to drive the compressor, a significant amount of heat is generated even at idle conditions. At idle conditions, a modern, large CTG releases approximately one third of the heat at idle conditions as it does at fill load. Thus, when starting up, CTGs generally produce much more heat than is needed.
When sufficient pressure has been raised in the HP drum, steam seals around the Steam Turbine Generator (STG) are set, and a vacuum is pulled using a steam jet air ejector (SJAE) or a mechanical vacuum pump. If a SJAE is used, sometimes the operator will take additional time to raise sufficient steam pressure to operate the SJAE. Once a partial vacuum is established, steam generated in the HRSG can be dumped into the condenser. A vacuum is also a prerequisite to starting the STG.
As pressure rises, the operator introduces steam to the main header connecting the HRSG to the STG. These pipes initially heat up rapidly because the steam condenses on the inside surfaces of these pipes. However, once the condensation stops, the pipes heat up much more slowly. Depending on the venting capacity of the throttle line, it can take thirty to sixty minutes to heat the line to the temperatures required to open the throttle valve and allow the STG to roll off turning gear.
Generally, before the STG can be rolled off, water chemistry in the HRSG steam drum has to be controlled to specified limits. This is achieved by flushing water through the drum until the desired chemistry is established. This is generally required by the time the drum pressure reaches 1000 psig, which can take thirty to sixty minutes.
The term “heat soak” as used in connection with this application is meant to refer to the process of gradually increasing the temperature of the steel in the HRSG and STG to minimize thermal stresses that can cause fatigue damage to the equipment. Heat is also applied to the inside surfaces of the shell of the steam turbine and the drum of the HRSG. If the temperature of the inside surface of the metal is significantly higher than that of the outside surface of the metal, the inside metal surface thermally expands more than the outside surface of the metal thereby placing the outside metal surface in tension, which can damage the metal. By gradually raising the interior temperature, however, heat has time to “soak” through the entire metal thickness thereby reducing stress.
Heat soaking is also used to reduce the temperature differential between the shell of the STG and the rotating rotor in the STG. To increase STG efficiency, the rotating parts of a STG are assembled very close to the stationary parts. If the rotating portions of the STG heat up faster than the stationary portions, the rotating portions will expand more and eventually rub against the stationary portions. Lastly, in some high temperature steam turbines, the steel has to change phase to become a more ductile form of steel that can resist the high stresses imposed at full load operation. This phase change also requires some time to occur.
Once the steam turbine rolls off turning gear, the startup sequence of the steam turbine is usually the critical path for plant startup. The steam turbine generally must be gradually started up and held at various speeds and loads to heat soak. Throttle and reheat steam temperature must also be regulated during this sequence. The steam turbine needs only limited amounts of steam while this is occurring, so any surplus steam must be vented to the atmosphere or dumped to the condenser. CTG loads are adjusted at this time to limit steam temperature and to minimize the amount of steam dumped to the condenser or the atmosphere.
Unfortunately, the CTG load level that will produce the desired steam temperature or flow will sometimes also result in extremely high levels of emissions. In most modern, large, combined cycle plants, Selective Catalytic Reduction (SCR) and CO (carbon monoxide) oxidation catalysts are used to control emissions. The SCR catalyst promotes reaction of ammonia with NOx compounds to produce nitrogen and water. The CO catalyst promotes oxidation to destroy CO and volatile organic carbons (VOCs). These catalysts are usually located downstream of the high-pressure evaporator. Because an evaporator is extremely effective in removing heat from the exhaust gas, the temperature of the flue gas leaving the evaporator section is normally only about 15° F. above the saturated steam temperature in the HP drum, and catalyst temperatures are effectively controlled by controlling drum pressure. These catalysts generally require a flue gas temperature of approximately 500° F. to destroy appreciable proportions of the pollutants present; thus, the HP drum pressure must reach approximately 600 psia before they will operate effectively. The SCR can sometimes require additional contact time because the aqueous ammonia consumed by the SCR must be vaporized in a packing tower which can require heat soaking time.
Additionally, thermal stresses during startups can cause significant damage to combined cycle plants. Metal fatigue is concentrated in two areas: in the metal in the drum of the high pressure evaporator and in the shell and blades of the high pressure section of the steam turbine. Because the high pressure steam drum operates at high pressures and temperatures and needs a large diameter, it has a thick shell. If the drum heats up quickly, the inside of the drum will become much hotter than the perimeter. This sets up thermal stresses that, over time, can cause the drum to fail in fatigue. A large steam turbine can encounter even higher stresses because it operates at almost the same pressure as the steam drum and at much higher temperatures. Modern reheat steam turbines commonly reach metal temperatures of about 1000° F. As the blades in the front end of the steam turbine approach this temperature, the metal of which they are constructed undergoes a phase change. The steam turbine cannot be heavily loaded until this change occurs, and this in turn requires that the steam turbine be carefully loaded and/or heat soaked.
Because of all of these constraints, starting up a modern combined cycle plant can require a significant amount of time and expense. For example, a typical, recently constructed 750 megawatt (MW) combined cycle plant required three to ten hours to start up. Start up costs, net of power sales, typically range from $25,000 to $80,000 per startup. In modern operating practice, combined cycle plants are started and stopped much more frequently than in the past.
When power plant operation was controlled by utilities, plant starts were carefully apportioned among the plants owned by a utility to avoid wearing out individual plants. Today, however, plants are essentially controlled by futures traders; when the cost of electricity is high, power traders want the plant to start as quickly as possible. When the cost is low, power traders want to shut down the plants immediately. Thus, plants can be required to start up daily and must be designed to withstand thermal stresses caused by these frequent startups over the typical twenty-year life of the plant. With deregulation, sales prices for electricity can vary significantly from hour to hour while fuel costs are generally constant over such a relatively short time period. Starting during off-peak periods is expensive because the revenue earned when the electricity is sold at a low price may not recoup the fuel cost. On the other hand, starting during on-peak periods takes time, which reduces the revenue earned during on-peak operation.
Another consideration of growing importance is emissions during startup. Regulators have recently become aware of the high amounts of pollutants emitted during startup. A typical large CTG, such as the Westinghouse 501F, emits several pounds per hour of CO during steady-state operations, but over 500 pounds of CO during each startup. Regulators are beginning to require operators to account for these emissions by buying offsets (i.e., transferring permission to pollute from one operator to another). This is often difficult and expensive to do as these offsets can be in short supply. In addition, regulators are beginning to impose restrictions on startup emissions. To reduce these emissions, the HP drum should be maintained at 600 psig or higher to allow the catalysts to function effectively. None of the prior art combined cycle systems is effective in controlling emissions, in part because of the failure to operate at optimum conditions during startup.
The prior art in this field discloses numerous techniques for reducing startup durations and costs. One previous method of controlling steam production during startup, for example, was to install a startup damper between the CTG and the Heat Recovery Steam Generator (HRSG). Such a startup damper was intended to vent a portion of the CTG exhaust so that the HRSG only produced the amount of steam needed to start up the Steam Turbine Generator (STG). Such plants, and the general control system therefor, are described in Taber et. al., U.S. Pat. No. 4,437,313, which is incorporated herein by reference.
This technique is not commonly used in recently designed combined cycle plants, however, because recent environmental regulations generally prohibit discharging CTG exhaust to the atmosphere without passing it through SCR and oxidation catalyst treatment steps. If a bypass stack is used, it would also have to be equipped with SCR and CO catalysts, which would further increase costs. In addition, the SCR treatment step takes enough time to operate that the bypass operation would likely be finished before the SCR was operational.
Combined cycle plants commonly incorporate auxilary boilers. The steam in the boiler is used to set the steam seal around the steam turbine, retaining the condenser vacuum after the plant is shut down. Steam from this boiler can also be injected into the HP drum to maintain pressure in the drum. Because the HP evaporator tubes are such efficient radiators of heat, it is difficult to maintain drum pressure much above 15 psig, even if the outlet stack is provided with a damper to retain heat. The prior art in this field recognizes several techniques for introducing heat to an HRSG, for example as taught in Kuribayashi et al., U.S. Pat. No. 4,282,708, which is incorporated herein by reference.
Several other patents describe methods of warning steam turbines during startup, such as U.S. Pat. No. 5,473,898 (Briesch). U.S. Pat. No. 5,412,936 (Lee et al.) describes a method of modulating steam temperature to the STG. U.S. Pat. No. 4,576,124 (Martens et al.) describes a method of startup using a trailing HRSG. U.S. Pat. No. 5,029,443 describes a method of starting up a power plant using a start-up gas turbine and startup HRSG to produce steam and electricity to start the plant. This patent does not rout exhaust gas from the start-up gas turbine to the HRSGs of the other plants, so it has the disadvantages of a plant with an auxiliary boiler. Each of the foregoing patents is incorporated herein by reference.
Some previous combined cycle plants have manifolded their CTG exhausts together allowing exhaust to flow from one HRSG or another. An example of this art was the AES Placerita plant located in California. This plant utilized two identical CTGs feeding a common duct to feed two HRSGs. One HRSG was designed for power generation while the other was designed to produce steam to increase oilfield production. Four isolation dampers were utilized to isolate each CTG and HRSG from the common duct. This design, however, has the disadvantages that the cross-over duct cannot be isolated, that a small CTG is not available for startups, and that the cross-over duct (being large enough to pass the entire flow of a large CTG) imposes a large pressure drop penalty compared to a small duct required for heat-up only.
Lastly, several prior art combined cycle plants have used fresh-air firing configurations. Forced-draft fresh-air firing utilizes an isolation damper or slide gate capable of isolating the CTG from the HRSG; an external fan capable of blowing combustion air into the inlet of the HRSG; another isolation damper or slide gate capable of isolating the fan from the inlet of the HRSG; and a burner located within the HRSG or between the fan and the fan isolation valve.
During normal operation of a forced-draft fresh-air fired HRSG, the CTG supplies hot exhaust to the HRSG; the CTG damper is open; the fan damper is closed; and the fan is off. During fresh-air firing, the CTG is off, the fan and burner are on, the CTG damper is closed, and the fan damper is open. Fresh-air firing HRSGs are designed so that the fresh-air system operates when the CTG is unavailable, or occasionally, when the CTG goes out of service.
Another, more unusual, form of fresh-air firing is known as induced draft fresh-air firing. This variant utilizes a slide gate or damper isolating the CTG from the inlet of the HRSG; an inlet damper capable of drawing fresh air directly into the inlet of HRSG; an internal duct-burner; and an induced draft fan located between the HRSG and its stack, capable of drawing the entire flow of the HRSG into the stack.
During normal operation of this configuration, the CTG and induced draft fan are in operation, the CTG duct is open and the fresh air duct closed, and the duct-burner is off or operating at part load. When the CTG goes out of service, the fresh air damper opens, the CTG duct closes, and the burner firing rate increases to maintain HRSG steam production. This configuration is usually only installed when the steam supply from the HRSG must not be interrupted, even if the CTG trips.
Both of the above-described fresh-air firing configurations are designed to supply steam from the HRSG in the event that the CTG trips, the CTG is unavailable, or it is uneconomic to operate the CTG. It appears from available literature and commercial knowledge that no fresh-air firing systems have ever been designed or used to help start-up a combined cycle system. A fresh-air fired system designed to start up a combined cycle system would almost certainly have to be designed differently. At a minimum, it would have a smaller fan and burner because full flow and heat release is not desired during startup.
It has now been found, however, that methods and apparatus according to this invention for controlling the operation of a combined cycle power plant during startup and part-load operations overcome the above-described limitations and disadvantages of conventional operation in whole or at least in part.